Nonequilibrium photoresponse behavior has been investigated for YBa2Cu3O7-x (YBCO) granular films to 8 mm microwave radiation under various bias currents and magnetic fields. The measurements reveal that the nonequilibrium photoresponse mode occurs only in the tail region of the resistance transition curve R(T) from the normal to the superconducting state, where transportation behavior of the granular superconducting film is found to be characterized by the Kosterlitz-Thouless (KT) phase transition model. Based on the KT model, the photoresponse mechanism has been interpreted in terms of the depinning process of the unbinding vortices, which are generated from the decoupling process of the vortex-antivortex pairs by current, and are held at the intrinsic pinning sites of the granular high-T-c superconducting films at low temperature. Under the co-action of the bias current and the incident microwave photons, these unbinding vortices will be driven out of the pinning center, creating viscous motion in the Josephson junction array system. An analytical result of the unbinding vortices density n(T,I) induced by applied current has been worked out based on the model of two-dimensional Josephson junction arrays that is employed as a model system for the YBCO granular films. The distribution of the n(T,I) is found to be analogous to that of the photoresponse measured in the temperature region of 2/3T(KT)展开▼
